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Um eletrodo íon seletivo foi construído e avaliado para determinação de íons amônio em águas naturais. O eletrodo consiste de uma pasta de carbono com o compósito SiO 2 /ZrO 2 / fosfato-NH 4 + obtido pelo processo de sol-gel. O eletrodo exibe uma resposta sub-Nenstiana para concentrações entre 7,7×10-7 e 4,0×10 -2 mol L -1 e um limite de detecção de 1,58×10 -7 mol L -1 (8,5×10 -3 ppm). A resposta do eletrodo foi rápida (1 minuto). Os coeficientes de seletividade K pot A,B para os diversos íons geralmente presentes em águas naturais foram determinados pela aplicação do matched potential method. O método potenciométrico com o eletrodo íon seletivo foi validado pelo método de Berthelot (método padrão) pela determinação de íons amônio em águas naturais. O eletrodo íon seletivo provou ser satisfatório para o controle de qualidade de águas naturais.Ion selective-electrode was constructed and evaluated for determination of ammonium ions in natural waters. The electrode consists of paste carbon with the composite SiO 2 /ZrO 2 /phosphate-NH 4 + obtained through sol-gel process. The electrode exhibits a sub-Nernstian response for ammonium concentrations between 7.7×10 -7 and 4.0×10 -2 mol L -1 and a detection limit of 1.58×10 -7 mol L -1 (8.5×10 -3 ppm). The electrode response for ammonium was fast (1 minute). The selectivity coefficients K pot A,Bfor several ions usually present in natural waters were determined applying the matched potential method. The potentiometric method with the ion selective electrode was validated by the Berthelot method (standard method), through the determination of ammonium ions in natural waters. The ion selective-electrode proved suitable for routine quality control of natural waters by potentiometry.Keywords: ammonium ion sensor, potentiometric sensor, natural waters IntroductionThe presence of ammonium ions in rivers and lakes can be associated in most cases to pollution caused by livestock, intensive agriculture and mainly, nowadays, to liberation of domestic sewage in courses of superficial waters in an indiscriminate way and in larger and larger amounts.Ammonium ion sensor is a useful tool in environmental pollution control, 1 food and clinical analysis 2-4 and other industrial applications.5 Among the ion sensors, ammonium sensor is also used more frequently as potentiometric biosensor in which enzymes or microbial cells are immobilized on the surface of the sensor. 6,7 Due to the many applications of the ammonium ion sensors, design of solid-state sensor for ammonium and its fabrication through simple procedure are desirable. 8Inorganic ion exchangers are characterized by high thermal stability and resistivity towards strong oxidizing agents and mineral acids. Recently the SiO 2 /ZrO 2 / phosphate composite was prepared by the sol-gel process, 9 which allies the good properties of ionic exchange of the zirconium(IV) phosphate 10,11 in a matrix of mixed oxide, SiO 2 /ZrO 2 high and homogeneously dispersed. This composite presents high superficial area, phosphate groups on the surface o...